Quite a lot of applications make use of suitable dye-based filters or of dyes, present in layers of materials or in subjects wherein well-defined wavelength ranges of irradiating sources should be blocked or selectively transmitted. So in the following fields it is recognized that dyes are crucial in order to fully provide advantage of their characteristic properties.
One particular filter application is related with displays and other equipment utilized in military, sports, transportation activities, whether or not in combination with night vision equipment. Displays and equipment conventionally utilize a filter to accomodate the night vision equipment as in aviation applications, wherein color displays are used with night vision imaging systems. Such displays provide visual information to captains, pilots, drivers and operators of ships, aircraft and vehicles and the viewer of color display often wears goggles at the same time while information from the color display is observed. Typically such goggles are sensitive to light between 425 nm and 1000 nm. So at 600 nm the sensitivity rapidly increases and reaches a peak at 760 nm in order to see objects which cannot ordinarily be seen by the naked eye. Compatibility problems with cockpit displays may however arise and are more particularly related with contrast reduction. This may lead to limited viewability and impaired object recognition as fine details cannot be observed. Conventional avionic displays designed to be used with night vision imaging systems are moreover generally restricted to narrow emission, such as single color displays (e.g. green colored displays). It is clear that interference with night vision imaging systems should be avoided. Difficulties are further related with highlighting and differentiating large amounts of information if the display is restricted to one single color. In another conventional avionic system color displays include a night vision imaging system filter, wherein the color display operates in a low luminance night vision mode and a daylight mode. A filter such as a liquid crystal display is provided therefore between the light source and an optical shutter. In daylight mode the displays use a second light source to provide light directly through the optical shutter without traversing the filter, so that the second light source is positioned so that its light is not provided through the night vision imaging system filter. Conventional filters are generally comprised of glass or other material supplemented by thin dielectric film coatings that attenuate infrared emissions or transmissions: thin multilayer dielectric films are typically used in order to provide a sharp cutoff, starting between 600 nm and 630 nm and additionally an absorptive substrate is used to attenuate longer wavelength emissions. In another attempt to attenuate such radiation, flexible emissive displays such as organic light-emitting diodes (OLEDs) are developed, but manufacturing is difficult. Furtheron thin film dielectric stacks are reflective in high ambient or daytime lighting and thereby reduce the contrast ration of the display. So a conventional thin film night vision imaging system filter may reflect as much as 50% of the light that strikes it at 630 nm wavelength.
In another application field such as in the field of color photographic applications, filter dyes are used as described e.g. in DE-A 19542239, in EP-A 0 696 758 and in U.S. Pat. No. 4,770,984. Important therein are filters used during film exposure, while taking photographs (e.g. in order to filter ultraviolet rays) as well as while exposing the negative image, thus providing color corrections in order to provide the most natural colors for the ultimate print as delivered to the customer. Analog as well as digital signals, sensitive in different wavelength ranges are envisaged therein and require filtering of undesired side-absorptions.
In another application field a diagnostic imaging system for reading out stimulable phosphor screens, plates or panels advantageously makes use of dye-based filters as (red) stimulating radiation should be unambiguously separated from light emitted upon stimulation of energy stored therein after exposure to X-ray energy in medical imagang systems (diagnostic imaging such as e.g. mammography, chest imaging, non-destructive testing applications, in portal and verification radiographic oncology systems and the like) as an array of e.g. transducer elements arranged for detecting said light emitted upon stimulation and for converting said light into a signal representation of said image should not be hit by stimulation radiation. Filtering means for preventing light emitted by said source of stimulation light from being detected by said transducer elements should, in other words, be characterized in that a ratio of transmission at the stimulating emission wavelength of said source of stimulation light in the wavelength range from 500 nm to about 850 nm and transmission of stimulated light in the wavelength range between 300 nm and 500 nm is as low as possible. This is particularly important when the detector is a CCD array having the highest quantum efficiency in the red region. The stimulation light can only be filtered away in an efficient way when the wavelength of the light emitted upon stimulation is quite different from the green or red stimulation light, i.e., that there is no or a only negligible overlap between the stimulation radiation spectrum and the stimulated emission radiation spectrum. In favor of customer-friendly handling or manutention in a medical radiographic environment, wherein a lot of phosphor plates or panels are exposed and read-out (processed) one after another, even if processed in a random order, it is recommended that detection of the blue light, emitted after photostimulation, proceeds with filters transmitting blue light for all screens or panels, without the need to change filters inbetween consecutive readings. For practical reasons it would thus be desirable to make use of only one, same filter for all of the different plates scanned in one, same scanning unit, in applications requiring an optimized image quality as well as in applications requiring ordinary image quality, and, more particularly in favor of cost reduction. Use of only one scanner would thus be highly appreciated for different types of plates.